This invention relates to type printers typically used in calculators, cash registers, and the like, and more particularly to type printers for high speed and two-color printing.
As compared to printers of the serial type, printers having a type drum are generally employed for high speed printing by simultaneous printing of multiple columns within the same row of type. Market demand for multicolor, high speed printing has significantly grown in recent years.
Type drum printers, such as disclosed in Japanese Laid-Open Patent Publication No. 61-164865 and shown in FIG. 22, print one line at a time by rotating the print drum through one revolution. A type drum 3 in such printers is restricted from moving in the column direction and is driven by a step motor 60. The printer is equipped with a reset pulse detector 62 for detecting a reference phase of step motor 60. A type sheet 4 is wrapped around the type drum. The printer is supported by a frame 1 and includes an ink roller 61 for depositing ink onto type sheet 4. The recording medium is fed along a platen 22. The rotary force generated by step motor 60 is coupled to paper feed gear 23 for advancing the recording medium along platen 22.
Another conventional type drum printer, such as disclosed in Japanese Laid-Open Patent Publication No. 57-181883, obtains a timing pulse from a detector which is responsive to rotation of a type group (e.g., type drum). The type drum of the printer is driven and stopped by application of a force thereto.
Demand has recently increased for smaller, more compact type drum printers. Generally, the motor in the type drum printer is located outside the type drum as disclosed in Japanese Patent Publication No. 57-181883.
There are several drawbacks associated with conventional type drum printers. For example, the type on the type drum is limited to their respective column positions in printing along a given print line of the recording medium. Only one color of ink can be applied to the type on the type drum. Multicolor printing at any desired column is theoretically impossible. A reset pulse detector is required for detecting the reference phase of the step motor. The step motor is expensive. Complicated speed adjustment is required to achieve high speed printing. Many different demands are made on the control circuitry of the printer. The inefficiency of the step motor results in high current consumption. The complicated speed adjustments, demands made on the control circuitry, and inefficient step motors lead to expensive drive circuits.
During type selection, rotation of the type drum and operation of a detection mechanism are halted. The detection mechanism generates timing pulses used by the type drum for rotation of the latter. Consequently, the hammer drive during the printing process (i.e., powering of print electromagnet) cannot be performed in synchronism with these timing pulses. Timing for powering of the print electromagnet must be based on a reference clock signal rather than these timing pulses and is generated by the control circuitry and software. An increased load on the control circuit results. The reference clock frequency also must be highly accurate to ensure a stable energization of the print electromagnet. An increase in associated circuit costs results.
Vibration or rotational fluctuation is also typically encountered in conventional type drum printers when the type drum is stopped and started. Vibration of the type during printing results which degrades the print quality. Rotational fluctuations immediately after driving the type drum can cause incorrect next type selection. The vibrational and rotational fluctuations are aggravated at high printing speeds. The speed at which printing occurs while maintaining acceptable print quality is therefore undesirably limited.
The size of the type drum printer is also limited by the room required within the printer housing to accommodate the type drum and motor and the selection mechanism which is separately positioned within the printer. Furthermore, noise generated by the motor is not muffled by the type drum since the motor is positioned outside the type drum.
It is therefore desirable to provide a compact, relatively quiet, low power, high speed type drum printer which can be built and operated at low cost. Use of a step motor, reset pulse detector and other expensive parts should be avoided.